Staging: easycap: fix sparse warnings for module parameters

easycap_main.c:34:5: warning: symbol 'easycap_debug' was not declared. Should it be static?
easycap_main.c:36:5: warning: symbol 'easycap_gain' was not declared. Should it be static?

These two variables actually were declared in several places.
The variables are used in several files.
I've fixed "easycap_debug" so it gets declared in one place only and included properly.
For "easycap_gain" made it static and I created added a
->gain member to the easycap struct.  This seems cleaner than using a
global variable and later on we may make this controlable via sysfs.

Cc:Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>